<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            width: 100px;
            height: 100px;
            border: 1px solid red;
        }

        #d {
            margin: 0 auto;
            width: 400px;
            height: 400px;
            border: 1px solid red;
            position: relative;
        }

        #d1 {
            position: absolute;
            top: 10px;
            left: 10px;
        }
    </style>
</head>

<body>

    <div id="d">
        <div class='a' id="d1">div1</div>
        <div class='a' id="d2">div2</div>
        <div class='a'>div3</div>
    </div>
</body>

</html>